It is well known that adhesion to the surfaces of materials is improved by the exposure to a corona discharge. Thus corona treatment has been used to treat the surfaces of thermoplastic materials to improve their adhesion to printing inks, paints, coatings and bodies of other materials.
Many methods for the continuous corona discharge surface treatment of thermoplastic materials have been employed wherein the material continuum is passed through an air gap between stationary and roller electrodes. The stationary electrode is typically a bar or cluster of bars and the relatively large roller electrode is coated with a dielectric coating. A high voltage, of the order of 20 KV at 10 kHz, is typically impressed across the electrodes. A corona arc discharge is developed in the gap and produces surface treatment of the thermoplastic material continuum which results in the promotion of excellent adhesion properties on the surface of the treated continuum. However, the provision and maintenance of such dielectric coatings on such roller electrodes which support the material to be treated present a number of problems which result in operational difficulties.
The dielectric roller coating is a major factor in good performance of the corona treatment process. Several qualities are sought for the reasons indicated below.
1. The capacitance per unit area must be high and this requires a high .English Pound./t ratio where .English Pound. is the dielectric constant and t is the thickness. Corona power is directly proportional to capacitance per unit area.
2. The buffer must have a high dielectric strength (i.e., volts/mil=E.sub.max ; an electric field) since this surface may experience the full applied electrode voltage and large working voltages correspond to large corona powers.
3. The roller coating should be capable of dissipating the heat generated, unaffected by ozone and oxides of nitrogen, and mechanically tough.
Catastrophic failure of a roller can result in costly production losses. Replacement of a roller and shipment of the roller out of a plant for recoating is expensive. In some applications, the dielectric coating that is generally deposited on the roller electrode can instead be deposited on the stationary electrode. This reduces the more expensive coating of the roller electrode and the cost associated with the replacements of the roller. In either case the temperature of the surface of the dielectric coated electrode during high power levels of operations (exceeding 4 or 5 kilowatts per square meter) could exceed 100.degree. C. or even approach or exceed the melting point of the material being treated.
It is an object of the present invention to provide a duplex coated corona electrode that has a reduced stabilized operating temperature when used at high power levels of operations.
It is another object of the present invention to provide a duplex coating for a corona roller electrode that will enable the electrode to operate at a reduced temperature for higher power levels and higher speeds than single coated roller electrodes.